[MINOR] Update docs for functions.scala to make it clear not all the built-in functions are defined there

The title summarizes the change.

Author: Reynold Xin <[email protected]>

Closes apache#21318 from rxin/functions.

Author: rxin

sql/core/src/main/scala/org/apache/spark/sql/functions.scala

@@ -39,7 +39,21 @@ import org.apache.spark.util.Utils
 
 
 /**
- * Functions available for DataFrame operations.
+ * Commonly used functions available for DataFrame operations. Using functions defined here provides
+ * a little bit more compile-time safety to make sure the function exists.
+ *
+ * Spark also includes more built-in functions that are less common and are not defined here.
+ * You can still access them (and all the functions defined here) using the `functions.expr()` API
+ * and calling them through a SQL expression string. You can find the entire list of functions for
+ * the latest version of Spark at https://spark.apache.org/docs/latest/api/sql/index.html.
+ *
+ * As an example, `isnan` is a function that is defined here. You can use `isnan(col("myCol"))`
+ * to invoke the `isnan` function. This way the programming language's compiler ensures `isnan`
+ * exists and is of the proper form. You can also use `expr("isnan(myCol)")` function to invoke the
+ * same function. In this case, Spark itself will ensure `isnan` exists when it analyzes the query.
+ *
+ * `regr_count` is an example of a function that is built-in but not defined here, because it is
+ * less commonly used. To invoke it, use `expr("regr_count(yCol, xCol)")`.
  *
  * @groupname udf_funcs UDF functions
  * @groupname agg_funcs Aggregate functions